Naturally occurring hydrated metal aluminum silicates are called zeolites and are well known in the art as synthetic absorbents. The most common of these zeolites are sodium alumino zeolites. Zeolites consist basically of a threedimensional framework of SiO.sub.4 and AlO.sub.4 Tetrahedra. The tetrahedra are cross-linked by the sharing of oxygen atoms so that the ratio of oxygen atoms to the total of the aluminum and silicon atoms is equal to two or 0/(Al+Si)=2. The electrovalence of each tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, a sodium ion. This balance may be expressed by the formula Al.sub.2 /Na.sub.2 =1. The spaces between the tetrahedra are occupied by water molecules prior to dehydration. The main products of these types are known in the art as zeolite A, zeolite X, and zeolite Y.
Zeolites A, X, and Y may be distinguished from other zeolites and silicates on the basis of their X-ray powder diffraction patterns and certain physical characteristics. The composition and density are among the characteristics which have been found to be important in identifying these zeolites.
The basic formula for all crystalline sodium zeolites may be represented as follows: EQU Na.sub.2 O:Al.sub.2 O.sub.3 :xSiO.sub.2 :yH.sub.2 O
wherein the values for x and y fall in a definite range. The value x for a particular zeolite will vary somewhat since the aluminum atoms and the silicon atoms occupy essentially equivalent positions in the lattice. Minor variations in the relative numbers of these atoms does not significantly alter the crystal structure or physical properties of the zeolite. For zeolite A, an average value for x is about 1.85 with the x value falling within the range 1.85.+-.0.5. For zeolite X, the x value falls within the range 2.5.+-.0.5.
The formula for zeolite A may be written as follows: EQU 1.0.+-.0.2Na.sub.2 O:Al.sub.2 O.sub.3 :1.85.+-.0.5SiO.sub.2 :yH.sub.2 O
The formula for zeolite X may be written as follows: EQU 0.9.+-.0.2Na.sub.2 O:Al.sub.2 O.sub.3 :2.5.+-.SiO.sub.2 :yH.sub.2 O
The formula for zeolite Y may be written as follows: EQU 0.9.+-.0.2Na.sub.2 O:Al.sub.2 O.sub.3 :4.5.+-.1.5SiO.sub.2 :yH.sub.2 O
wherein y may be any value up to 6 for zeolite A, any value up to 8 for zeolite X, and any value up to 9 for zeolite Y.
In zeolites synthesized according to the preferred art procedure, the ratio sodium oxide to alumina should equal one. But if all the excess sodium present in the mother liquor is not washed out of the precipitated product, analysis may show a ratio greater than one, and if the washing is carried too far, some sodium may be ion exchanged by hydrogen, and the ratio will drop below one. It has been found that due to the ease with which hydrogen exchange takes place, the ratio for zeolite A lies in the range of ##EQU1## Thus the formula for zeolite A may be written as follows: EQU 1.0.+-.0.2Na.sub.2 O:Al.sub.2 O.sub.3 :1.85.+-.0.5SiO.sub.2 :yH.sub.2 O
Zeolite A is disclosed and claimed in U.S. Pat. No. 2,882,243, entitled "Molecular Sieve Adsorbents," issued to Robert M. Milton on Apr. 14, 1959. This patent discloses a process for producing zeolite A wherein a sodium-aluminum-silicate water mixture is prepared having a water to sodium oxide ratio of from 35:1 to 200:1, a sodium oxide to silica ratio of from 0.8:1 to 3:1, and a silica to alumina ratio of from 0.5:1 to 2.5:1. This mixture is maintained at a temperature of from 20 to 175 degrees Celsius until zeolite A is formed.
The particle size of a zeolite affects the X-ray diffraction pattern of that zeolite. When the particle size of a zeolite is reduced, the intensities of peak heights in the zeolite's X-ray diffraction pattern is also reduced. The X-ray diffraction pattern does not change, except that the intensity of each peak height is reduced.
The identity of a zeolite of small particle size can be determined by comparison of its X-ray diffraction pattern with that of a standard zeolite. The patterns should match, except that the intensities of the peak heights of the pattern of the small particle size zeolite will be smaller than the intensities of the peak heights of the pattern of the standard zeolite.
The particle size of zeolites is discussed in Breck, D. W. Zeolite Molecular Sieves. N.Y., John Wiley & Sons, 1974, pgs. 384-388. TP159.M6B7. He states that particle sizes of the individual crystals of zeolite range from 1 to 10 microns. He shows the particle size distribution of a typical zeolite sodium A powder having a weight average diameter of 2.78 microns and shows a histrogram of the particle size distribution of a zeolite A preparation. From the histrogram it can be seen that less than 35% of the particles have a diameter of less than 2 microns.
The particle size of zeolites is also discussed in Meier and Uytterhoeven Molecular Sieves, 1973, pgs. 169-178. This book shows a relationship between the crystal diameter and the water to sodium oxide ratio. It also shows the influence of the silica source on crystallization time.
Zeolite A and its method of preparation is also described in the following United States Patents wherein the invention resides in the method of preparation. U.S. Pat. Nos. 2,982,612; 3,058,805; 3,101,251; 3,119,659; 4,041,135;
The oxide ratios for zeolite A production in the above patents is as follows:
______________________________________ PRIOR ART OXIDE RATIOS FOR MAKING ZEOLITE A U.S. Water/ Sodium Oxide/ Silica/ Tempera- Pat. No. Sodium Oxide Silica Alumina ture ______________________________________ 2,882,243 35-200 0.8-3 0.5-2.5 20-175 2,982,612 130-300 0.3-1 4-6 60-110 3,058,805 25-200 1-3 0.5-1.3 20-175 3,058,805 35-200 0.8-3 1.2-2.5 20-175 3,058,805 ? ? 0.5-4.5 80 3,101,251 35-200 1.3-2.5 0.8-3 20-120 3,119,659 20-100 0.5-1.5 1.6-2.4 20-175 4,041,135 35-200 0.8-3 0.5-2.5 70-180 ______________________________________
From the prior art, one would assume that zeolite A could not be made from a reaction mixture having a water to sodium oxide molar ratio of less than 35:1 and a silica to alumina molar ratio greater than 2.4:1. Moreover, none of the above patents teaches a method of forming zeolite A of small and uniform size having a high magnesium carbonate exchange capacity.
The literature also indicates that the apparent pore diameter of zeolite A is between 3.6 and 4.0 angstroms, depending on temperature. At liquid nitrogen temperature (-195.4 degrees Celsius), the pore diameter is smallest and prevents nitrogen from being adsorbed into the crystal. For example, as pointed out by Breck, D. W. Zeolite Molecular Sieves, New York, John Wiley & Sons, 1974, pages 634-639, at 635;
"NaA adsorbs C.sub.2 H.sub.4 (slowly) and CH.sub.4, .sigma.=3.9 and 3.8A, respectively. At low temperatures, it does not adsorb N.sub.2. The apparent pore diameter is 3.6 to 4.0A, depending on temperature. The explanation for this variation has been based upon a process of activated diffusion. It has also been postulated that thermal vibration of the oxygen ions, and cations, in the zeolite lattice surrounding the apertures is responsible." PA0 "The activated sodium zeolite A adsorbs water readily and adsorbs in addition somewhat larger molecules. For instance, at liquid air temperatures it adsorbs oxygen but not appreciable amounts of nitrogen as shown below for a typical sodium zeolite A sample. PA0 (a) forming an aqueous solution of sodium aluminate; PA0 (b) forming an aqueous solution of sodium silicate; PA0 (c) mixing said sodium aluminate and said sodium silicate solutions at a temperature of 40.degree. to 120.degree. C.; PA0 (d) reacting said mixed sodium silicate and sodium aluminate at a temperature slightly higher than said mixing temperature, the reaction mixture having the following molar ratios of components: PA0 (e) continuing the reaction at these molar ratios to form the zeolite while controlling the molar ratios and reaction time to produce a fine particle size zeolite having an average particle size of less than 2 microns in diameter; and PA0 (f) recovering the zeolite.
Further, as pointed out in Milton U.S. Pat. No. 2,882,243, at column 13;
______________________________________ Weight per- Partial cent Tempera- pressure adsorbed Adsorbate ture (.degree.C.) (mm. Hg) on Na.sub.2 A ______________________________________ Oxygen -196 100 24.8 Nitrogen -196 700 0.6 ______________________________________